In recent years, in a field of dampers (shock absorbers) used in a vehicle suspension system, various types of variable damping force dampers that can variably control the damping force in steps or continuously have been developed. In the past, variable damping force dampers of a mechanical type were common in which a flow area of an orifice was controlled by using a motor, solenoid and the like, but in recent years, variable damping force dampers of a current control type have been developed in which a magnetic fluid or magneto-rheological fluid (MRF) is used as an operating fluid and an electric current supplied to a magnetic fluid valve is increased/decreased to control an apparent viscosity of the operating fluid.
In a vehicle equipped with a variable damping force damper of a current control type (simply referred to as a damper hereinafter), it is possible to improve the maneuverability and riding comfort of the vehicle by variably changing the control current of the damper within a prescribed range (e.g., 0-5 A) and thereby changing the damping force of the damper depending on operating conditions of the vehicle (see, for example, United States Patent Application Publication No. 2006/0047387). For instance, during a turn of the vehicle in which the vehicle body can roll in left and right directions due to an inertial force (lateral acceleration) resulting from the lateral movement of the vehicle, the control current of the damper can be set to a higher value so that the damping force of the damper is increased in accordance with a differentiation value of the lateral acceleration, to thereby suppress an excessive rolling of the vehicle body. Also, during a travel of the vehicle on a road having small surface irregularities in which the wheels of the vehicle can move up and down in short intervals, the control current is set to a lower value to reduce the damping force of the damper according to an actual stroke speed of the damper, to thereby suppress transmission of the up-down movements of the wheels to the vehicle body via the suspension or to ease impact from the wheels to the vehicle body.
In the above described damping force control technique, when the differentiation value of the lateral acceleration becomes large (and hence the target damping force becomes high) during a slalom run of the vehicle, for example, it becomes difficult for the damper to undergo telescopic movements in response to irregularities of the road surface. Particularly, when the differentiation value of the lateral acceleration exceeds a certain value, the control current of the damper can be fixed to its upper limit value (e.g., 5 A) irrespective of increase/decrease in the actual stroke speed of the damper. As a result, the reduction of damping force in response to increase in the stroke speed (or easing of the impact from the wheels) cannot be carried out, and this can deteriorate the riding comfort.